This invention relates in general to electrophotographic imaging members, and more specifically, to positively and negatively, preferably positively charged electrophotographic imaging members with a single electrophotographic photoconductive insulating layer and processes for forming images on the member. More specifically, the present invention relates to a single layered photoconductive imaging member useful in electrostatic digital, including color, process, and which members contain a charge generation layer or photogenerating layer comprised of a photogenerating component, such as a photogenerating pigment, dispersed in a matrix of a hole transporting and an electron transporting binder, and in embodiments a protective overcoat, such as a polymer layer. The electrophotographic imaging member layer components, which can be dispersed in various suitable resin binders, can be of various thicknesses, however, in embodiments a thick layer, such as from about 5 to about 60, and more specifically, from about 10 to about 40 microns, and yet more specifically, from about 15 to about 40 microns, is selected. This layer can be considered a dual function layer since it can generate charge and transport charge over a wide distance, such as a distance of at least about 50 microns. Also, the presence of the electron transport components in the photogenerating layer can enhance electron mobility and thus enable a thicker photogenerating layer, and which thick layers can be more easily coated than a thin layer, such as about 1 to about 2 microns thick.
The expression “single electrophotographic photoconductive insulating layer” refers in embodiments to a single electrophotographically active photogenerating layer capable of retaining an electrostatic charge in the dark during electrostatic charging, imagewise exposure and image development. Thus, unlike a single electrophotographic photoconductive insulating layer photoreceptor, a multi-layered photoreceptor has at least two electrophotographically active layers, namely at least one charge generating layer and at least one separate charge transport layer.
A number of known electrophotographic imaging members are comprised of a plurality of other layers such as a charge generating layer and a charge transport layer. These multi-layered imaging members in some instances also can contain a charge blocking layer and an adhesive layer between the substrate and the charge generating layer. Further, an anti-plywood layer may be included in the imaging member. Complex equipment and valuable factory floor space are usually needed to manufacture multi-layered imaging members. In addition to presenting plywooding problems, multi-layered imaging members often encounter charge spreading which degrades image resolution. The anti-plywood layer can be a separate layer or be part of a dual function layer. An example of a dual function layer for preventing plywooding is the use of a charge blocking layer or an adhesive layer. The expression “plywood” refers, for example, to the formation of unwanted patterns in electrostatic latent images caused by multiple reflections during laser exposure of a charged imaging member. When developed, these patterns resemble plywood. Multi-layered imaging members are also costly and time consuming to fabricate because of the many layers that need to be formed.
Another problem encountered with multilayered photoreceptors comprising a charge generating layer and a charge transport layer is that the thickness of the charge transport layer, which is normally the outermost layer, tends to become thinner due to wear during image cycling. The change in thickness can cause changes in the photoelectrical properties of the photoreceptor. Thus, to maintain image quality, complex and sophisticated electronic equipment and software management are usually encountered in the imaging machine to compensate for the photoelectrical changes, which can increase the complexity of the machine, the cost of the machine, the size of the footprint occupied by the machine, and the like. Without proper compensation of the changing electrical properties of the photoreceptor during cycling, the quality of the images formed can degrade because of spreading of the charge pattern on the surface of the imaging member and a decline in image resolution. High quality images can be important for digital copiers, duplicators, printers, and facsimile machines, particularly laser exposure machines that demand high resolution images. Moreover, the use of lasers to expose conventional multilayered photoreceptors can lead to the formation of undesirable plywood patterns that are visible in the final images.
Attempts have been made to fabricate electrophotographic imaging members comprising a substrate and a single electrophotographic photoconductive insulating layer in place of a plurality of layers such as a charge generating layer and a charge transport layer. However, in formulating single electrophotographic photoconductive insulating layer photoreceptors many problems need to be overcome including acceptable charge acceptance for hole and/or electron transporting materials from photoelectroactive pigments. In addition to electrical compatibility and performance, a material mix for forming a single layer photoreceptor should possess the proper rheology and resistance to agglomeration to enable acceptable coatings. Also, compatibility among pigment, hole and electron transport molecules, and film forming binder is desirable.